The present invention relates to an improved electrical resistance welder and, more particularly, to a reduction in the weight and reactance and an increase in efficiency of a resistance welding gun.
Electrical resistance welding is, of course, well known. Resistance welders are frequently used in the fabrication of vehicles to weld together parts of the vehicle such as floor pans, fenders, roofs, hoods, doors, frames, etc.
Many types of portable resistance welding guns have been designed to assist in the welding of large body portions of vehicles. These portable welding guns allow the welding electrodes to be taken to the workpiece.
One problem in a resistance welder is that each structural member, i.e., each arm of the welding gun, is inherently both a resistance and a reactance. Furthermore, in a welding gun having a wide throat, the air gap adds a substantial reactance of the welding circuit. In fact, it is often the case that in a resistance welder, the welding gun itself provides a resistance and a reactance which far exceeds the resistance and reactance of the load. In other words, the resistance and reactance of the equipment far exceeds the resistance and reactance of the material being welded.
In a conventional resistance type welding system, normally less than 10% of the total power requirement is used to produce the weld and the remaining 90% of the total power is needed to overcome the resistance and reactance of the welding gun itself plus any resistance and reactance of the cable used to interconnect the welding gun to the power transformer. Thus the power loss, resulting from a current drain, in the transfer of the welding current to the electrodes is proportional to the resistance and reactance of the throat of the welding gun and of the arms of the welding gun. Furthermore, there is also a substantial power loss in the cable itself when a cable is used between the welding gun and the power transformer.
While a basic objective in a resistance welder is to conserve energy by the reduction of line current, there are various conflicting subproblems which occur. Specifically, the minimum line current necessary for welding is related to the load current needed for welding in proportion to the transformer "turns ratio". To minimize line current, a higher "turns ratio" is needed. However, in order to provide a sufficient secondary voltage to overcome the total impedance of the system, a lower "turns ratio" is needed. But once a lower "turns ratio" is provided, a higher line current becomes necessary.
Thus, the desire to reduce line current has been frustrated by the necessity of a sufficiently high line current in conjunction with a sufficiently low "turns ratio" to provide not only the necessary power for welding but also the necessary secondary voltage to overcome the impedance of the welding system.
In my prior U.S. Pat. No. 4,233,488, of Nov. 11, 1980, I have provided a first solution for minimizing the impedance of the welding gun. The present invention provides yet another solution to the aforementioned problem.
Specifically, in considering the prior art resistance welders, it has been customary to provide a cooling member through which a coolant is flowed for the purpose of cooling the transformer of the welding gun. The cooling member must be thermally conductive, to draw the heat from the transformer, and a cooling fluid or coolant is flowed through the cooling member. Typically, copper tubes are used as the cooling member, because of the high thermal and electrical conductivity of copper. However, the use of a cooling member, particularly a metal cooling member, increases not only the weight but also the resistance and reactance of the welding gun.